LM358 Room Noise Detector with LED Indicator

Description:

One LED Indicates 50dB,70dB and 85dB. Useful to detect too noisy environments

This circuit is intended to signal, through a flashing LED, the exceeding of a fixed threshold in room noise, chosen from three fixed levels, namely 50, 70 & 85 dB. Two Op-amps provide the necessary circuit gain for sounds picked-up by a miniature electret microphone to drive a LED. With SW1 in the first position the circuit is off. Second, third and fourth positions power the circuit and set the input sensitivity threshold to 85, 70 & 50 dB respectively.

Current drawing is <1mA with LED off and 12-15mA when the LED is steady on.

Useage:

1. Place the small box containing the circuit in the room where you intend to measure ambient noise.
2. The 50 dB setting is provided to monitor the noise in the bedroom at night. If the LED is steady on, or flashes bright often, then your bedroom is inadequate and too noisy for sleep.
3. The 70 dB setting is for living-rooms. If this level is often exceeded during the day, your apartment is rather uncomfortable.
4. If noise level is constantly over 85 dB, 8 hours a day, then you are living in a dangerous environment.

Parts: 

R1	:10K 1/4W Resistor
R2,R3	:22K 1/4W Resistors
R4	:100K 1/4W Resistor
R5,R9,R10	:56K 1/4W Resistors
R6	:5K6 1/4W Resistor
R7	:560R 1/4W Resistor
R8	:2K2 1/4W Resistor
R11	:1K 1/4W Resistor
R12	:33K 1/4W Resistor
R13	:330R 1/4W Resistor
C1	:100nF 63V Polyester Capacitor
C2	:10µF 25V Electrolytic Capacitor
C3	:470µF 25V Electrolytic Capacitor
C4	:47µF 25V Electrolytic Capacitor
D1	:5mm. Red LED
IC1	:LM358 Low Power Dual Op-amp
Q1	:BC327 45V 800mA PNP Transistor
MIC1	:Miniature electret microphone
SW1	:2 poles 4 ways rotary switch
B1	:9V PP3 Battery
	Clipfor PP3 Battery

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